This application relates to pressurized-gas energy storage batteries, and, more particularly, to controlling the charging of nickel-hydrogen batteries.
Rechargeable batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. The energy storage battery may be recharged after it has delivered the useful power, leading to a succession of charging and discharging cycles. Familiar examples of the rechargeable energy storage battery are the lead-acid battery used in automobiles and the nickel-cadmium battery used in various portable electronic devices. Another type of battery having a greater storage capacity for its weight is the nickel oxide/pressurized hydrogen energy storage battery, an important type of which is commonly called the nickel-hydrogen battery and is used in spacecraft applications.
The nickel-hydrogen battery includes a series of active plate sets which store an electrical charge electrochemically and later deliver that charge as a useful current. The active plate sets are packaged within a hermetic pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the energy storage battery. Each plate set includes a positive electrode, a negative electrode, and a separator between the two electrodes, all soaked with an electrolyte. In a typical nickel-hydrogen battery, a number of plate sets are supported on a core under a compressive loading, with a gas screen between each plate set and with electrical connector leads extending to each electrode of each plate set. The gas screen provides a gas channel from the hydrogen electrode to the gas space outside the stack. A single nickel-hydrogen battery delivers current at about 1.3 volts, and a number of the batteries are usually electrically interconnected in series to produce current at the voltage required by the systems of the spacecraft.
During the operation of the nickel-hydrogen battery, hydrogen is produced within the pressure vessel during the charging portion of the charging/discharging cycle. The pressure within the pressure vessel is monitored and used as an indicator of the state of charge of the energy storage battery and thence as the control parameter for the charging apparatus. That is, increasing pressure is taken to correspond to a increasing state of charge, within the limits of operation of the energy storage battery. However, the energy storage battery changes temperature during its periods of service, so that the measured pressure may change as the temperature of the hydrogen increases, without any change in the state of charge. The measured pressure may therefore be adjusted to account for changes in gas pressure associated with temperature changes rather than changes in the state of charge, prior to its use as a control parameter.
With this approach, overcharging of the storage battery is sometimes observed. Such overcharging leads to overheating of the storage battery, and possibly to its premature failure. There is a need for an understanding of the causes of such overcharging, and for an improvement to the control system to avoid this problem. The present invention fulfills that need, and further provides related advantages.